The present invention is directed generally to magnetic recording media and devices incorporating the media and, more particularly, to zinc (Zn) containing layers for use with cobalt or cobalt alloy based magnetic layers in the formation of magnetic recording media and the recording devices used in data storage.
As the home, office, transportation vehicle, business place and factory becomes more automated and electronically connected, and as electronic devices and appliances such as computers, communication devices, electronic games, entertainment systems, personal data assistants transportation systems, vehicles, manufacturing tools, shop tools, and home appliances become more sophisticated there is, and will be, an ever-increasing demand for low cost magnetic recording media with greater storage capacity. In order to keep the storage devices unobtrusive and inexpensive each product generation must store more information in a smaller space. Hence there are ever increasing demands for technical improvement of the media and storage systems.
In general this means that not only must media attributes be improved, but that the transducer used to record or retrieve the data must be capable of resolving extremely small distances and changes in the media. With the exception of a few optical storage systems, such as those based upon holographic processes or multi-photon processes, this means that for all future storage systems the transducer must be in extremely close proximity to the media. This is certainly the case for all modern magnetic recording systems, where the ability to resolve the recorded data falls off exponentially with distance between the transducer and the media. This is the case in perpendicular, isotropic or longitudinal magnetic recording systems such as used in hard disk, magnetic tape, and floppy disk systems. This is even the case in the proposed near field optical recording systems as well as for the envisioned future x-y addressable systems such as those that might be base upon micro-machined silicon structures. What this means for the magnetic media is that in order to place the transducer close to the magnetic media, so that better resolution is possible, it is best if the magnetic media layer is very thin. Furthermore, there must not be much physical space allocated to be between the surface of the magnetic layer and the transducer, such as might be used for a physical wear layer or lubricant. However, some of these structures are required to actually have a working media. Clearly the entire structure must also be extremely smooth to allow the transducer to approach the media. Hence, since the early 1980s, as shown in the market place, there has been a movement toward thin film technology for the recording media. Thin media has enabled the rapid advances toward higher linear and areal recording densities.
Most commercially available thin film magnetic media is based upon hexagonal closed packed (HCP) cobalt alloys. This is because the HCP Co crystalline phase possesses both a relative large saturation magnetization, Ms, that can be adjusted by alloying, and a large uniaxial magnetocrystalline anisotropy energy density, Ku, which is necessary to achieve high coercivity, Hc. Certainly for hard disk drive storage, the linear storage density is directly related to the coercivity and so to the anisotropy energy density. Due to the statistical nature of the playback signal a minimum signal to noise ratio (SNR) is required from the recording system in order to guarantee accuracy in reading back the recorded data. For a number of years the SNR in data storage systems has been limited by the statistical nature of the media, as opposed to the other sources of noise such as Johnson noise of the electronics or transducer. Since the media is granular in nature and the data bit cell size is inversely related to the areal recording density the media magnetic switching unit size determines the maximum possible SNR and so areal recording density that can be supported. Even in what is referred to as continuously exchange coupled media the bit cell wall location is determined by localized fluctuations in the media properties and so its storage capability is controlled by the granularity of these fluctuation locations. It is also true in optical recording systems such as the Compact Disk ReWriteable (CD-RW) whether they be based upon magneto-optic, MO, or phase change, PC, media. Basically, the power SNR is proportional to the number of switching units contained in a given data cell. For example for typical, modern hard disk drives approximately 100 to 1000 switching units are required per data bit cell to achieve a sufficient SNR. With the SNR fixed for a given required system data retrieval reliability, this implies that the size of the switching units must be decreased to increase the areal recording density. Unfortunately, the magnetic switching unit size is not always as small as might be thought by measuring the magnetic crystalline grain size. For example, if two magnetic grains, or particles, directly touch then they commonly become magnetically exchange coupled via the material""s electronic wave functions. This means that the two grains tend to switch as a single larger unit. This reduces the number of particles or switching units in a data bit cell and causes the media noise to be worse. Because the two grains are usually not oriented in the same direction it also tends to lower the overall anisotropy energy density for the switching unit to a smaller value than that predicted for a single crystal grain. Also, if the crystalline grains are not perfect in structure, including crystalline defects or defects in the surface quality, the Ku value will be less than predicted from bulk crystalline measurements. Lowering Ms can lower Hc, but if Ms does not decrease as fast as Hc increases then it must be due to other effects, such as improved effective Ku. Both of these effects may compromise the coercivity of the media and limit the recording density. Hence, it is a major objective in the construction of magnetic media to have small, isolated magnetic switching units with sufficient anisotropy energy density to provide a coercivity to record short wavelength data bit patterns.
One measurement method developed to determine whether or not magnetic grains are exchange coupled is referred to as the delta M, dM, method. This bulk magnetic measurement compares the difference between the initial magnetization process, from the demagnetized state, to the reversal process from the saturated magnetic state. If there is little fundamental difference between these two magnetization processes then the magnetic particles are said to be non-interacting and the dM values will approach zero. If the dM values are positive then for Co based longitudinal media magnetic grains are said to be exchange coupled. Hence, a near zero or negative dM value is a reasonable measure indicating the media grains are de-coupled.
For longitudinal disk magnetic recording, the recording performance must look the same at all locations around the concentrically recorded tracks on the disk. This usually implies that the disk media has a set of magnetic particles that are randomly oriented with respect to the circumferencial location on the disk. It is the function of the media materials and media construction and process to achieve these attributes and more. Historically, a mechanical grooving in a circumferencial direction of the disk surface, sometimes called mechanical texture, has been used to create a slight orientation of the magnetic direction of the magnetic particles. However, the maximum that has been achieved is quite small. This orientation is measured by determining the ratio of magnetic properties when measure along the circumference direction versus the radial direction on the disk. The ratio of the coercivity, Hc, or the remanent magnetization, Mr, values when measured along these two directions, is referred to as the orientation ratio, O.R. Typical ratios obtained by using mechanically grooved substrates are only about 1.1 to 1.2.
Recently it has been noted that, if the product of the magnetic grain volume and the anisotropy energy density is too low, compared to thermal vibration energies, kBT, the magnetization of a switching unit may spontaneously reverse due to thermal fluctuations. That is, the magnetization of each switching unit may thermally decay to a random state rendering previously stored data useless. Hence, for a given anisotropy energy density, there is a minimum volume for the magnetic switching unit for which the data will be stable over a reasonable time period. This then can place a lower limit on the magnetic grain size and hence an upper limit on the possible areal recording density. At first assessment one would think he could simply choose a magnetic material with a higher anisotropy energy density to avoid this instability. However, if Ku is too high then the recording transducer cannot reverse the magnetization of each switching unit (i.e. record or erase data) in a short period of time. Hence, it is desirable to have a magnetic media with each magnetic switching unit being as small as possible to provide an acceptable SNR, but of high crystalline quality so as to achieve the maximum intrinsic Ku value. If more than one crystalline grain switches together as a unit the media appears to have a smaller number of grains, a larger distribution of magnetic particle sizes, and a higher media noise level. Furthermore, the Ku value appears to be compromised. Hence, it is very desirable that the magnetic switching unit size be determined by the crystalline grain dimension and this is achieved by isolating the individual crystalline grains. Small improvements in the media microstructure can evidently provide very large improvements in the thermal stability. This is evidenced by the fact that the media with an O.R of only slightly higher than one, where one represents total in plane random orientation of the grains, can be more thermally stable. However, some hard disk media substrates, such as glass and glass ceramics, cannot easily be mechanically grooved circumferencially and so cannot benefit from this method to improve thermal stability. Hence, new methods to improve the media microstructure to narrow the particle size distribution and to promote smaller, isolated magnetic grains with defect free perfect crystalline order are needed.
The structure of a typical thin film hard disk media is multilayered and includes a substrate at its base covered by an underlayer structure, a magnetic layer structure and optionally, an overlayer at the top. The overlayer is usually coated with a very thin overcoat layer and an organic lubricant. This overcoat is usually made of a hard form of carbon and provides mechanical, as well as some corrosion protection.
The underlayer structure maybe composed of multiple underlayers with multiple functions, such as, seed layers to initiate the film growth morphology; followed by underlayers used to set the grain size, size distribution and to set or improve the crystalline texture; and intermediate layers usually used to transition from one crystal structure to another or from one lattice atomic spacing to another. In most cases the layers are each epitaxially grown on the previous layer, grain on grain, and the final surface of the underlayer structure is usually used to induce epitaxial growth in the following magnetic structure. The underlayer structure can also provide the function of physical isolation of other layers from the substrate. In some cases, where the substrate is electrically insulating, they can provide a conductive base so that electrical contact can be made between the new media substrate surface and the deposition system.
The magnetic layer is the main body on which the magnetic bits are recorded. State of the art recording media is comprised of cobalt or cobalt alloy-based magnetic films having a hexagonal close packed, HCP, crystal structure and the underlayer is used to induce this crystal structure and to prevent the magnetically soft FCC phase of Co from developing. In its simplest form the underlayer structure has been composed of a bcc Cr or CrX alloys. Where the some of the alloying elements, X, that have been reported in the literature have been (X=C, Mg, Al, Si, Ti, V, Co, Ni, Cu, Zr, Nb, Mo, La, Ce, Nd, Gd, Tb, Dy, Er, Ta, and W). Other non-Cr elements reported are Ti, W, Mo and NiP. While there would appear to be a number of underlayer materials available, in practice only a very few materials work well enough to meet the demands of the industry. Among them, the most successful and often used underlayers are Cr and Cr alloys, where elements such as V, Mo, Ti, and W where these elements have been used to modify the lattice constant to better match that of the Co-alloy. U.S. Pat. No. 4,652,499 discloses efforts to improve the underlayer of longitudinal magnetic media by adding vanadium (V) to Cr to change its lattice constant and thereby to promote a better lattice matching between the HCP Co alloy, such as CoPt or CoPtCr, and the BCC CrV underlayer. Likewise, Mo and Ti have also been used to expand the Cr lattice constant.
This modification of the lattice constant is especially needed when the large Pt atom is included in the Co-alloy. However, more recently, especially on insulating substrates such as glass and glass ceramics rather than the traditional NiP coated AlMg substrates, a NiAl or FeAl bcc derivative crystal structure film has been introduced between the substrate and the Cr underlayer to induce a unique (112) crystal texture and to reduce the grain size. This texture then induces the very desirable Co (10.0) unicrystal texture. This is described in U.S. Pat. No. 5,693,426, which is incorporated herein by reference. Hence, as used herein a xe2x80x9cbccxe2x80x9d structure shall include both body centered cubic and body centered cubic derivative crystal structures, such as the B2, DO3, and the L12 crystal lattices. The notation 10.0, for example, and other crystalline texture notations showing the number and dot notation used throughout are equivalent to the conventional number and bar overwrite notations. For example, 10{overscore (1)}0 is equivalent to 10.0, and 11{overscore (2)}0 is equivalent to 11.0. Both systems of notation are well known in the art.
As just described, for high density longitudinal recording, in plane orientation has heretofore been achieved by grain-to-grain epitaxial growth of the HCP Co alloy thin film on a bcc underlayer. The polycrystalline Co-based alloy thin film is deposited with its c-axis, the [00.2] axis, either parallel to the film plane or with a large component of the c-axis in the film plane. It has been shown by K. Hono, B. Wong, and D. E. Laughlin, xe2x80x9cCrystallography of Co/Cr bilayer magnetic thin filmsxe2x80x9d, Journal of Applied Physics 68 (9) p. 4734 (1990), that BCC Cr underlayers promote grain-to-grain epitaxial growth of HCP Co alloy thin films deposited on these underlayers. The heteroepitaxial relationships between BCC Cr and HCP Co which bring the [00.2]Co axis down or close to the film plane are (11.0)Co//(002)Cr, and (10.1)Co//(110)Cr. This work was based upon the sputter deposition process on to glass or NiP plated Al substrates and the different Co/Cr epitaxial relationships prevail as the Cr texture varied for different deposition processes. While the relationship (10.0)Co//(110)Cr was also reported, it has since been shown by growing extremely well oriented Cr films on various single crystal Si substrates that this relationship does not occur. See Wei Yang et al., xe2x80x9cEpitaxial Ag templates on Si(001) for bicrystal CoCrTa media,xe2x80x9d Journal of Applied Physics, vol. 81, No. 8, p. 4370 (Apr. 15, 1997). The Cr (110) texture always results in a Co (10.1) texture. It has since been shown that to obtain the Co(10.0) texture a BCC (112) texture is desired. To form a good media the Co alloy needs to form the HCP crystalline structure. Each of these BCC textures tend to promote the Co HCP. However, the (10.0) Co //Cr(112) provides for the most defect free Co grain with c-axis fully in-plane and so it the most desirable. The second most desirable is the (11.0)Co//(002)Cr as the c-axis is fully in-plane, but the grains suffer from defects and from binary variants resulting in a lowered anisotropy energy density. The (10.1)Co//(110)Cr relationship is the least desirable as the grains suffer from both defects, variants and a c-axis that is oriented at about 28 degrees with respect to the media plane. To obtain a good BCC structure, which promotes the formation of the HCP structure, the Cr underlayer should be thicker than about 50 xc3x85.
Some longitudinal media products use the Cr alloy in direct contact with the Co alloy while others place a non-magnetic HCP CoCr alloy as an intermediate layer between the Cr alloy and the Co alloy. The 30 to 40 atomic percent Cr in the CoCr alloy renders it non-magnetic, but its HCP crystal structure can provide the transition between bcc to HCP crystal structures without increasing the thickness of the magnetic layer structure.
Likewise, to achieve perpendicular high density recording media, the perpendicular orientation of the Co c-axis with respect to the film plane has usually been obtained by grain-to-grain epitaxial growth of the HCP Co alloy thin film to an oriented HCP underlayer of (00.2) crystalline texture. Ti and Ti90Cr10at % are often cited as the best seed layers. for this purpose, although other seed layers, such as Pt, CoO/Pt and non-magnetic HCP CoCr35at % have been used to induce this structure. See xe2x80x9cDevelopment of High Resolution and Low Noise Single-layered Perpendicular Recording Media for High Density Recordingxe2x80x9d, IEEE Trans. Magn., Vol. 33, no. 1, p. 996-1001 (January 1997); xe2x80x9cCompositional separation of CoCrPt/Cr films for longitudinal recording and CoCr/Ti films for perpendicular recordingxe2x80x9d IEEE Trans. Magn., Vol. 27, no. 6, part 2, pp. 4718-4720 (1991); xe2x80x9cProperties of CoCrTa Perpendicular films prepared by sputtering on Pt underlayerxe2x80x9d, J. MMM, Vol. 155, no. 1-3, pp. 206-208 (1996); IEEE Trans. Magn. Vol. 32, no. 5, pp. 3840-3842 (September 1996); IEEE Trans. Magn. Vol., 30, no. 6, pp. 4020-4022 (Nov. 1994); and, xe2x80x9cDevelopment of High Resolution and Low Noise Single-layered Perpendicular Recording Media for High Density Recordingxe2x80x9d, IEEE Trans. Magn. Vol. 33, no. 2, pp. 996-1001) (January 1997).
For a period of time the longitudinal magnetic layer could have been composed of more than one magnetic layer. However, as the recording density has increased and the need to make the layer thin has become more important the magnetic layer structure is usually reduced to a single or at most two magnetic layers or a single magnetic layer in contact with an antiferromagnetic layer. As noted in Li-Lien Lee, David E. Laughlin and David N. Lambeth, xe2x80x9cCrMn Underlayers for CoCrPt Thin Film Media,xe2x80x9d IEEE Transactions on Magnetics, Vol. 34 (4), July 1998, pp. 1561-1563 an antiferromagnetic layer coupled to the magnetic layer may improve the coercivity and thermal stability of the media by effectively increasing the volume of the magnetic grain without lowering the SNR. Cobalt-based alloys as opposed to pure cobalt are commonly used in longitudinal and perpendicular magnetic media for a variety of reasons. For example, non-magnetic elements such as Cr are commonly bulk doped into the magnetic film to lower the magnetization. This increases the coercivity, but does not improve thermal stability. This lowering of Ms is especially important in perpendicular media where the demagnetization energy associated with the magnetic moment of the alloy must be less than the magneto-crystalline anisotropy energy in order for the magnetization to be oriented perpendicular to the media film plane. The same technique is used in longitudinal magnetic media to lower the flux transition demagnetization energy, resulting in a shorter flux transition length and, hence, higher recording densities. Even more importantly, however, non-magnetic elements are introduced into the Co-alloy to limit the magnetic exchange coupling between cobalt grains. It is believed that preferential diffusion of elements such as Cr, P, or B from the bulk of the magnetic grain to the grain boundaries during film growth help to isolate the individual grains by reducing the magnetic exchange coupling between grains. This then results in a significantly lower media noise. For example, Deng et al. found that the addition of small amounts of Ta to CoCr alloys resulted in the increased Cr diffusion to the grain boundaries. See Youping Deng, David N. Lambeth, and David E. Laughlin, xe2x80x9cStructural Characteristics of Bias Sputtered CoCrTa/Cr Filmsxe2x80x9d, IEEE Transactions on Magnetics, Vol. 29, no. 5, September 1993, pp. 3676-3678.
The recording attributes, such as signal level, media noise level, linear and areal recording density, and thermal stability are directly dependent upon the magnetic media properties. Hence, proper magnetic media properties, such as anisotropy (Ku), coercivity (Hc), coercive squareness (S*), remanant magnetization (Mr) remanant squareness (S), and delta M values are crucial to the recording performance. For a given magnetic alloy composition, these magnetic properties are primarily dependent on the microstructure of the film. For thin film longitudinal magnetic recording media, the desired crystalline structure, or texture, of the modem Co alloys is hexagonal close packed (HCP) with a uniaxial crystalline anisotropy directed along the crystalline c-axis located predominately in the in the plane of the film (i.e., in-plane). Usually, the better the in-plane c-axis crystallographic texture, the higher the coercivity of the Co alloy thin film used for longitudinal recording. High coercivity is required to achieve a high remanence. A high anisotropy is required to achieve a high degree of thermal stability of the recorded patterns. Likewise for perpendicular magnetic recording media the desired crystalline structure of the Co alloys is HCP with the uniaxial anisotropy and crystalline c-axis must lying perpendicular to the film plane. Due to thermal stability issues and surface imperfections, for very small grain size coercivity increases with increased grain size. Large grains, however, results in greater media noise and if the grains become too large domain walls can be nucleated internally to lower the coercivity dramatically. Or if the large grains are actually composed of exchange coupled clusters of smaller grains then both the media noise and the coercivity degrades. Exchange coupled magnetic grains tend to switch their magnetization together, instead of independently, when a magnetic field is applied. Thus, a group of these exchange-coupled grains can act effectively as a large magnetic grain during recording, which significantly increases the transition noise of the recording media. Hence, as mentioned previously, isolation of the magnetic grains is very critical in obtaining low noise and a high coercivity thin film magnetic recording media. Also, because of the variation in the magnetocrystalline anisotropy direction in going from the region of one grain to another, granular exchange coupling promotes magnetic reversal via domain wall motion across these magnetic exchange coupled grain boundaries, resulting in lower coercivity. Hence, there is a need to achieve high coercivities without the increase in noise associated with large grains or clusters of grains.
As mentioned the use of Cr and B in the Co alloys is a widely adopted approach used to achieve grain isolation via segregation at the grain boundaries for a variety of Co alloys. However, a relatively high Cr concentration in CoCrX alloys is required for significant Cr segregation. But as noted by N. Inaba, M. Futamoto, and A. Nakamura, IEEE Trans. Magn., vol. 34, pp. 1558-60 (1998), the crystalline anisotropy energy density decreases with increased Cr content. This is undesirable as a high anisotropy energy density is required in order to obtain a high coercivity and is absolutely necessary to stabilized the recorded data against thermal decay. Hence, other methods of promoting grain boundary isolation are being explored. In fact, U.S. Pat. No. 5,993,956, xe2x80x9cManganese Containing Layer for Magnetic Recording Media,xe2x80x9d which is incorporated herein by reference, and J. Zou, D. E. Laughlin, and D. N. Lambeth, MRS Symp. Proc., vol. 517, pp. 217-22 (1998) discusses the use of a Manganese (Mn) containing layer in contact, or near contact, to the Co-alloy layer. These layers can be either under or on top of the magnetic layer and can be separated from the magnetic layer by other thin layers provided they can still diffuse to the magnetic layer interface. In these teachings, diffusion of the Mn along and to the Co-alloy grain boundaries tends to break the magnetic exchange coupling between the Co-alloy grains. Hence, allowing the grains to function magnetically as isolated units and not as large clusters of grains. However, other than Mn, few materials have been reported to be very effective at this type of grain isolation via grain boundary diffusion from another layer without degrading the magnetic properties of the magnetic layer.
Diffusion distances into Co can be estimated by the following temperature dependent equation:
X2=Dt
xe2x80x83Where:
D=D0exp(-Q/RT)
X2 is the square of the distance that a species diffuses,
D is called the diffusional coefficient,
t is the time spent at elevated temperature,
D0 is a constant for a given diffusing species,
R is the universal gas constant or 1.98719 cal/mole-K, and
Q is the activation energy for diffusion of the species into the Co grains.
From this diffusion equation one can see that the square of the distance of diffusion is only linear in time, but is thermally activated and so is an exponential in temperature (Kelvin). This means that the diffusion is a strong function of temperature so temperature plays a very important role in the diffusion process. A typical values for the activation energy for Mn diffusing in Co can be estimated from the data of Swalin and Martin (Trans AIME 206, 567ff, 1956). While these values are actually that for Mn diffusing in Cu, they will be similar to the values for Mn diffusing in Co since Cu and Co have similar atomic sizes. If the Mn is raised from 150xc2x0 C. (423xc2x0 K.) to 250xc2x0 C. (523xc2x0 K.) xc2x0C. the diffusion coefficient, D, increases 10 fold. Hence, Mn would diffuse 3.3 times as far for this 24% increase in temperature. This huge difference illustrates the strong effect of temperature on the diffusion. From the equation we can see that a comparable difference in the activation energy would have an equally resounding effect. Hence, since the activation energy for diffusion along grain boundaries is considerably less than that for diffusion into the bulk of a material, diffusion from an adjacent layer to another layer first takes place via the grain boundaries. For example for FCC metals the activation energy for diffusion along the grain boundaries is about xc2xd that of the activation energy for diffusion into the bulk. However, if the sample is left at an elevated temperature for sufficiently long time, diffusion into the bulk of the grain will also occur. Hence, the optimization of the processing temperature and time is necessary to obtain a preferential diffusion to the grain boundaries.
Clearly, under proper processing conditions it is possible to diffuse sufficient overlayer (top-layer) or underlayer materials to the magnetic layer grain boundaries while diffusing only small amounts into the bulk of the magnetic grains. Simply put, for a given time and temperature, the diffusion along the grain boundary is considerably more rapid that the diffusion through the bulk of the grain. Therefore, diffusion of the proper non-magnetic material may be used to obtain grain isolation while only causing a moderate decrease in the magnetic anisotropy energy density of the magnetic layer material. Obviously, the material chosen to provide the grain boundary exchange decoupling is important. When mixed in sufficient quantity with the Co alloy at the grain boundaries the resulting material should decouple the grains magnetically.
The need for lighter, smaller and better performing and less costly computers with greater storage density demands higher density recording media for use in hard disk drives, other magnetic storage devices, and other applications. It is an object of the present invention to meet those demands with a magnetic recording media having high coercivity, good thermal stability and low noise.
The present invention is directed to the use of Zn containing layers positioned between a substrate and a magnetic layer, in contact with the magnetic layer, or in proximate or near contact with the magnetic layer. The Zn containing layer provides a magnetic recording media having increased coercivity and lower media noise. Diffusion of the Zn to the magnetic layer grain boundaries provides enhanced magnetic grain isolation and improved magnetic recording properties. The Zn containing layer may be incorporated in an underlayer structure, in the magnetic layer structure, or in the overlayer. As used herein, reference to the Zn containing layer as being xe2x80x9cadjacent toxe2x80x9d the Co or Co alloy layer shall include any one of the foregoing structural positions (between a substrate and a magnetic layer, in contact with the magnetic layer, or in proximate or near contact with the magnetic layer to permit diffusion of Zn to the magnetic layer grain boundaries). The Zn need only be in close proximity to the Co or Co alloy layer to be effective at providing a diffusional source for the Co or Co alloy grain boundaries.
Accordingly, the present invention includes a magnetic recording medium comprising a substrate, a magnetic thin film structure forming a magnetic recording layer, and a Zn containing layer. The Zn containing layer includes at least one of Zn, a solid solution Zn alloy, and a Zn containing crystalline structure composed of at least one other element. The zinc containing layer is disposed adjacent the magnetic layer. The magnetic layer may have a magnetic c-axis oriented substantially parallel to the magnetic layer. Alternatively, the magnetic layer may have a magnetic c-axis oriented substantially perpendicular to the magnetic layer.
The Zn containing layer may be made of a material selected from the group consisting of Zn, or Zn in combination with at least one element from the group consisting of Ag, Au, Cd, Ce, Cr, Co, Cu, Eu, Fe, Gd, Mg, Mn, Ni, Nd, Pr, Pd, Tb, Ti, Y, Yb, and Zr.
The recording medium of the invention may further include an underlayer disposed between the substrate and the Zn containing layer. The underlayer is made of a material selected to promote epitaxial crystalline structure in the magnetic layer. The underlayer is preferably made of a material selected from the group consisting of Cr, CrV, CrMo, CrW, CrTi, NiAl, AlCo, FeAl, FeTi, CoFe, CoTi, CoHf, CoZr, NiTi, CuBe, CuZn, AlMn, AlRe, AgMg, and Al2FeMn2.
The recording medium of the invention may further include an intermediate layer disposed between the substrate and the Zn containing layer. The intermediate layer preferably includes a material selected to promote epitaxial crystalline structure in the magnetic layer. The intermediate layer may be made of a material selected from the group consisting of Cr, Cr alloys and a material having a BCC derivative crystalline structure and a lattice constant substantially comparable to Cr.
The recording media can be incorporated in a data storage device having rotating, translating, or stationary media for use in conjunction with magnetic transducing heads for the recording and reading of magnetic data, as well as other applications.
The magnetic recording medium of the invention preferably includes a Co or Co alloy magnetic layer, and a Zn containing layer. The Zn containing layer can be formed from pure Zn, or Zn and another element or elements from the group Ag, Au, Cd, Ce, Co, Cu, Eu, Fe, Gd, Mg, Mn, Ni, Nd, Pr, Pd, Tb, Ti, Y, Yb, and Zr. Unfortunately, there are few binaries of Zn that actually form a pure body centered cubic crystal structure. Hence, when the Zn containing layer is part of the underlayer structure, while it is preferred that it be in a BCC structure it will most often and in particular, need to be a B2 crystal structure. The most preferred B2 structures include AgZn, AuZn, BaZn, CuZn, MnZn, TiZn, CeZn, TiZn, NbZn, NdZn, NiZn, PdZn, and ZrZn where the lattice spacings of the (112) or (002) texture is chosen to match and so to induce the Co alloy (10.0) unicrystal or (11.0) bicrystal structure. For example, the CuZn is preferred when used with a large lattice constant Pt containing Co alloy as the lattice constant of CuZn is 0.295 nm or about 2.3% larger than Cr. Another set of Zn containing B2 crystal structure group includes a rare earth element, such as DyZn, ErZn, EuZn, GdZn, HfZn, LaZn, LuZn, PrZn, SmZn, TbZn, TmZn, Yzn, or YbZn. The rare earth-B2 crystalline structures are of particular interest because they almost always form line compounds. Hence, a slightly rich Zn rare earth combination results in a B2 crystal structure with excess Zn residing at the B2 grain boundaries, which are then readily available to diffuse to the magnetic layer structure grain boundaries. Even though Zn is insoluble in many BCC materials, the Zn would reside at the BCC grain boundaries and, hence, be readily available for diffusion. Hence, materials resulting from mixing Zn with BCC materials where the Zn is not soluble, but the BCC crystals are readily formed suffice as sources of Zn for diffusion. For example, Zn deposited with Cr or Cr alloys can provide Zn already at the underlayer grain boundaries and so physically aligned with the magnetic layer grain boundaries. Similarly, when an HCP intermediate layer is employed, Zn can be added to it. Just as with the insoluble BCC, the use of insoluble Zn in an HCP intermediate layer will perform a similar diffusion function. For example, Zn can be included in the non-magnetic CoCr alloys. Of course, it is clearly understood that it is possible to mix these materials, or to choose groups of three or more elements provided that at least one of them is Zn. Examples of three element B2 crystal structures containing Zn are Ni2SiZn, Cu2TiZn, Ni2TiZn, and Cu2ZrZn, the latter of which has a lattice constant approximately 3% larger than Cr.
Most preferably, the Co or Co alloy magnetic layer has an HCP structure and is deposited with its c-axis, the magnetic easy axis (the direction of the principle anisotropy energy density minimum), substantially parallel to the plane of the magnetic layer for longitudinal media and, for perpendicular media, substantially perpendicular to the plane of the magnetic layer.
The medium can further include additional layers in the underlayer structure, such as seed layers, underlayers, and intermediate layers. For example polycrystalline MgO has been taught as a preferred seed layer for longitudinal media in U.S. Pat. No. 5,800,931. The underlayers and/or intermediate layers used in addition to the Zn containing layer generally include materials having either an A2 structure, also known as a BCC crystal, or a BCC derivative, such as the B2, DO3, or the L21-ordered crystalline structure, disposed between the seed layer and the magnetic layer. Materials having an A2 structure are preferably Cr or Cr alloys, such as CrV, CrMo, CrW, or CrTi. Materials having a B2-ordered structure having a lattice constant that is substantially comparable to that of Cr, include NiAl, AlCo, FeAl, FeTi, CoFe, CoTi, CoHf, CoZr, NiTi, CuBe, CuZn, AlMn, AlRe, AgMg, and Al2FeMn2, and most preferably, FeAl or NiAl. Materials having a DO3-ordered structure include, for example, FeIIIAl, BeFeIII and Mg3La. A representative L21-ordered crystalline structure is AlNi2Ti. An intermediate Zn containing layer is preferably disposed between the underlayer and the magnetic layer. However, it should also be understood that a thin underlayer or intermediate layer placed between the Zn containing layer and the magnetic layer can significantly improve the preferential diffusion of the Zn to the magnetic layer grain boundaries by inhibiting diffusion to the bulk of the individual magnetic grains. The objective in this case is that diffusion to the magnetic layer grain boundaries can occur essentially only via the adjacent non-Zn containing layer grain boundaries. That is, in a well grown epitaxial structure the grain boundaries of one layer are predominately aligned with the grain boundaries of the next layer. Furthermore, it is desirable that the non-Zn containing layer have a very low or no Zn solubility. For example, Zn does not appear to dissolve into W or V and there is little evidence that it dissolves into Cr. When a non-Zn containing layer is placed between the Zn containing layer and the magnetic layer structure the thickness of non-Zn containing layer should be only comparable in thickness to the magnetic layer structure thickness. Preferably, the non-Zn containing layer thickness would be no more than 10 times the thickness of the magnetic layer structure thickness and most preferably less than the magnetic layer thickness. Clearly, the underlayer structure may be formed of multiple layers wherein each layer is a different one of the foregoing materials.
More than one magnetic layer can be incorporated in the media and it can also include one or more inner layers disposed between the magnetic layers. The inner layers are typically about 10 to 40 xc3x85 thick and composed of Cr or Cr alloy, but they can also be the Zn containing layer of the present invention. However, because it is desirable to keep the magnetic layer structure as thin as possible, it is preferred that the magnetic layer structure be composed of no more than two magnetic layers, and most preferably composed of a single layer.
The magnetic layer may be covered by an overlayer, which, in turn, may be covered by an overcoat. An organic lubricant is preferably added over the overcoat to reduce frictional wear of the media. The overlayer may be comprised of Zn or a Zn containing material.
The present invention includes a method of producing a magnetic layer containing grain boundaries on a recording substrate and then preferentially diffusing Zn to said grain boundaries. The method comprises disposing an underlayer structure to cause epitaxial growth of the said magnetic layer, disposing a magnetic layer, disposing a Zn containing layer adjacent to the magnetic layer, and diffusing Zn to the magnetic layer grain boundaries, which is preferably done by heating the Zn containing layer. Heating the Zn containing layer may occur after the deposition of the magnetic layer and Zn containing layer, or alternatively, before deposition of the magnetic layer. Zn is disposed in an amount sufficient to promote isolation of the magnetic grains. The method may further include interposing an intermediate layer between the Zn containing layer and the magnetic layer.
The present invention also includes a method for using a Zn containing alloy as a sputtering target for the production of magnetic recording media having a Zn containing layer, particularly thin film magnetic recording media as described herein.
The exact process of producing the media with an optimum Zn diffusion is dependent upon the vacuum system capabilities and optimization for both performance and manufacturing cost. In the latest modern single disk deposition systems, where one disk is coated with one material at a time, the rate of deposition can be very fast. For example, it is very common in these systems to deposit several tens of nanometers of material in only a few seconds. On the other hand, in pallet style deposition systems where several disks are coated with one layer at the same time (parallel deposition), it is common for the deposition rate to be somewhat slower. Likewise, in a deposition system designed for research or development studies, where versatility of the choice of materials is of importance, as opposed to the low cost manufacturing of media, the deposition of a single material layer may take several minutes. For this reason, the deposition temperature may be adjusted considerably to obtain a similar diffusion of material at the material microstructure level. While a deposition process, and material system, such as the ones described herein can be optimized for Zn diffusion during the steps of layer deposition, it is also the case that the diffusion of the materials may be more effective if done after all layers, or only some of the layers, have been deposited. This is especially true when the deposition processes are slow. One very effective approach to achieve this optimization is to perform a post deposition thermal anneal to achieve the desired level of diffusion. In this embodiment, the most preferred approach is to perform a higher temperature anneal to cause diffusion of Zn after both the Zn containing layer and the magnetic layer have been deposited.
Accordingly, the present invention provides magnetic recording media and data storage devices incorporating recording media having high coercivity and lower noise for use in hard disk drives and other data storage applications. These advantages and others will become apparent from the following detailed description.